U.S. patent number 6,335,546 [Application Number 09/364,768] was granted by the patent office on 2002-01-01 for nitride semiconductor structure, method for producing a nitride semiconductor structure, and light emitting device.
This patent grant is currently assigned to Sharp Kabushiki Kaisha. Invention is credited to Yuhzoh Tsuda, Takayuki Yuasa.
United States Patent |
6,335,546 |
Tsuda , et al. |
January 1, 2002 |
Nitride semiconductor structure, method for producing a nitride
semiconductor structure, and light emitting device
Abstract
A nitride semiconductor structure includes: a substrate having a
growth surface, a convex portion and a concave portion being formed
on the growth surface; and a nitride semiconductor film grown on
the growth surface. A cavity is formed between the nitride
semiconductor film and the substrate in the concave portion.
Inventors: |
Tsuda; Yuhzoh (Tenri,
JP), Yuasa; Takayuki (Nara-ken, JP) |
Assignee: |
Sharp Kabushiki Kaisha (Osaka,
JP)
|
Family
ID: |
16691602 |
Appl.
No.: |
09/364,768 |
Filed: |
July 30, 1999 |
Foreign Application Priority Data
|
|
|
|
|
Jul 31, 1998 [JP] |
|
|
10-216639 |
|
Current U.S.
Class: |
257/94;
257/E21.119; 257/E21.131; 438/42 |
Current CPC
Class: |
H01L
33/007 (20130101); H01L 33/20 (20130101); H01L
21/02378 (20130101); H01L 21/0242 (20130101); H01L
21/0243 (20130101); H01L 21/02458 (20130101); H01L
21/0254 (20130101); H01L 21/0262 (20130101); H01L
21/02639 (20130101); H01L 21/02647 (20130101); H01L
33/32 (20130101); H01S 5/32341 (20130101) |
Current International
Class: |
H01L
21/20 (20060101); H01L 21/02 (20060101); H01L
33/00 (20060101); H01S 5/323 (20060101); H01S
5/00 (20060101); H01L 033/00 () |
Field of
Search: |
;257/94,103
;438/42,44,46,47 ;117/45,902,913 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Sunakaw et al. (1997) "Thick GaN crystal growth with low defect
density by hydride vapor phase epitaxy" Ext. Abstract. (The
58.sup.th Autumn Meeting) J. Soc. of Appl. Phys. No. 1:266 2p-Q-15,
(English abstract enclosed herewith). .
Tanaka et al. (1997) "Reduced dislocation densities in
selectivity-grown nitride materials" Ext. Abstract. (The 58.sup.th
Autumn Meeting) J. Soc. of Appl. Phys. (1997) No. 1:265 2p-Q-14
(English abstract enclosed herewith). .
Usui et al. (1997) "Thick GaN Epitaxial Growth with Low Dislocation
Density by Hydride Vapor Phase Epitaxy" Jpn. J. Appl. Phys. 36(2),
No. 7B:L899-L902..
|
Primary Examiner: Chaudhuri; Olik
Assistant Examiner: Wille; Douglas A.
Attorney, Agent or Firm: Morrison & Foerster LLP
Claims
What is claimed is:
1. A light emitting device or a laser device comprising:
a nitride semiconductor structure including a substrate having a
growth surface, a convex portion and a concave portion being formed
on the growth surface, and a nitride semiconductor film grown on
the growth surface, and
a light emitting structure formed on the nitride semiconductor
structure, the light emitting structure including an active layer
comprising a nitride semiconductor, wherein the convex portion and
the concave portion of the nitride semiconductor structure comprise
a plurality of parallel grooves.
2. A light emitting device or a laser device according to claim 1,
wherein the plurality of grooves have a width b and a depth h such
that b.ltoreq.about 10 .mu.m and h.gtoreq.0.2.times.b, and wherein
adjoining ones of the plurality of grooves are spaced apart from
each other so as to retain a distance of about 20 .mu.m or less
between center lines thereof.
3. A light emitting device or a laser device according to claim 1,
wherein the plurality of grooves have a width b and a depth h such
that b.ltoreq.about 10 .mu.m and h.gtoreq.b, and wherein adjoining
ones of the plurality of grooves are spaced apart from each other
so as to retain a distance of about 20 .mu.m or less between center
lines thereof.
4. A light emitting device or a laser device according to claim 1,
wherein the plurality of grooves are formed along a <1-100>
direction of the nitride semiconductor.
5. A light emitting device or a laser device according to claim 1,
wherein the plurality of grooves are formed so as to extend along a
cleavage plane or an etching stable plane of the substrate.
6. A light emitting device or a laser device according to claim 1,
wherein the growth surface of the substrate of the nitride
semiconductor structure comprises a nitride semiconductor formed on
the substrate, and
the plurality of grooves are formed along a <11-20> direction
of the substrate semiconductor.
7. A light emitting device or a laser device according to claim 2,
wherein the growth surface of the substrate of the nitride
semiconductor structure comprises a nitride semiconductor formed on
the substrate, and
the plurality of grooves are formed along a <11-20> direction
of the nitride semiconductor.
8. A light emitting device or a laser device according to claim 3,
wherein the growth surface of the substrate of the nitride
semiconductor structure comprises a nitride semiconductor formed on
the substrate, and
the plurality of grooves are formed along a <11-20> direction
of the nitride semiconductor.
9. The light emitting device or laser device of claim 1, wherein
the nitride semiconductor includes at least one element selected
from the group consisting of B, As, P and Sb.
10. A light emitting device or a laser device comprising:
a nitride semiconductor structure including a substrate having a
growth surface, a convex portion and a concave portion being formed
on the growth surface, and a nitride semiconductor film grown on
the growth surface; and
a light emitting structure formed on the nitride semiconductor
structure, the light emitting structure including an active layer
comprising a nitride semiconductor;
wherein a cavity is formed between the nitride semiconductor film
and the substrate of the nitride semiconductor structure in the
concave portion.
11. A light emitting device or a laser device according to claim
10, wherein the convex portion and the concave portion of the
nitride semiconductor structure comprise a plurality of parallel
grooves.
12. A light emitting device or a laser device according to claim
11, wherein the plurality of grooves are formed along a
<1-100> direction of the nitride semiconductor.
13. A light emitting device or a laser device according to claim
11, wherein the plurality of grooves are formed so as to extend
along a cleavage plane or an etching stable plane of the
substrate.
14. A light emitting device or a laser device according to claim
11, wherein the growth surface of the substrate of the nitride
semiconductor structure comprises a nitride semiconductor formed on
the substrate, and
the plurality of grooves are formed along a <11-20> direction
of the nitride semiconductor.
15. The light emitting device or laser device of claim 11, wherein
the nitride semiconductor includes at least one element selected
from the group consisting of B, As, P and Sb.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a nitride semiconductor structure.
In particular, the present invention relates to: a nitride
semiconductor structure including a substrate for allowing crystal
growth and a high-quality nitride semiconductor grown on the
substrate for allowing crystal growth, the nitride semiconductor
having a different lattice constant or a different thermal
expansion coefficient from that of the substrate; a method for
producing such a nitride semiconductor structure; and a light
emitting device produced by employing such a nitride semiconductor
structure.
2. Description of the Related Art
Conventionally, nitride semiconductors have been employed as
materials for constructing blue light emitting diodes (referred to
as "blue LEDs") or blue laser diodes (referred to as "blue LDs"). A
nitride semiconductor is typically grown on a substrate by a
metal-organic chemical vapor deposition (MOCVD) method, a hydride
vapor phase epitaxy (HVPE) method, or a molecular beam epitaxy
(MBE) method. In general, when a semiconductor is grown on a
substrate, a substrate is used which is either of the same material
as the semiconductor to be grown thereon or has a lattice constant
and/or a thermal expansion coefficient similar to those of the
semiconductor to be grown thereon.
It is impossible in the state of the art to prepare an
appropriately-sized nitride semiconductor substrate which is of the
same material as an overlying nitride semiconductor. Accordingly, a
sapphire substrate, a SiC substrate, a spinel substrate, or the
like is used as a substitute for a nitride semiconductor substrate.
However, due to the large difference in lattice constant or thermal
expansion coefficient between a nitride semiconductor and a
sapphire substrate used as a substitute substrate, it is known that
a nitride semiconductor film which has been grown directly on a
sapphire substrate may contain threading dislocations at a density
on the order of 10.sup.9 to 10.sup.10 cm.sup.-2. Therefore, it has
been difficult by allowing crystal growth directly on a substitute
substrate to obtain satisfactory nitride semiconductor crystals,
i.e., nitride semiconductor crystals having substantially no
crystal defects or a substantially zero threading dislocation
density.
As used herein, a "threading dislocation" is defined as a
dislocation, particularly that occurring within a crystal or at an
interface between crystals, that reaches the surface of the
substrate.
Currently, a selective growth method is commonly adopted as a
method for producing a nitride semiconductor film directly on a
sapphire substrate because it is supposed to reduce the density of
crystal defects or threading dislocations.
Hereinafter, a conventional method for producing a nitride
semiconductor film will be described which utilizes selective
growth of a nitride semiconductor.
In a first step, a first layer of a nitride semiconductor is formed
directly on a sapphire substrate by using a MOCVD apparatus. In a
second step, a SiO.sub.2 layer is vapor deposited directly on the
first layer of nitride semiconductor by using a chemical vapor
deposition (CVD) method. In a third step, SiO.sub.2 layer is
processed so as to form a pattern having periodic openings by a
known lithography technique. In a fourth step, the sapphire
substrate which has undergone the third step is placed into a HVPE
apparatus so as to grow a second layer of nitride semiconductor
thereon. In accordance with this procedure, the density of
threading dislocations (which would cause deterioration in the
crystal quality) in the second layer of nitride semiconductor,
which has been grown in the fourth step, is reduced to about
6.times.10.sup.6 cm.sup.-2. See Proceedings of 58th Applied Physics
Association Lecture Meeting, 2p-Q-15 No. 1 (1997) p. 266"; or Jpn.
J. Appl. Phys. Vol. 36(1997) p. L899. The reduction in the
threading dislocation density is due to the selective growth of the
nitride semiconductor on the SiO.sub.2 masking pattern during the
third step. Specifically, the second layer of nitride semiconductor
which is grown directly on the masking pattern is more likely to
develop in the openings of the masking pattern than in the portions
where the SiO.sub.2 layer remains.
The initial growth of the second layer of nitride semiconductor
begins mainly in the openings. As the growth reaches the uppermost
level of the SiO.sub.2 layer, lateral growth begins so as to bury
the SiO.sub.2 masking layer, while the growth also continues along
the direction perpendicular to the substrate. This lateral growth
does not emanate from the underlying masking layer but rather from
the nitride semiconductor crystals grown in the openings, which
serve as growth cores. Therefore, the lateral growth is less
susceptible to lattice mismatching.
Although the threading dislocations that are generated within the
first layer of nitride semiconductor may intrude the second layer
of nitride semiconductor through the openings in the masking layer,
they are diverted by the lateral growth so as to proceed along the
lateral direction. Consequently, few threading dislocations reach
the uppermost surface of the nitride semiconductor, resulting in
crystals having a low threading dislocation density.
Alternatively, it is also possible to form a SiO.sub.2 masking
pattern directly on a sapphire substrate and selectively grow a GaN
monocrystalline film by MOCVD, as reported in Proceedings of 58th
Applied Physics Association Lecture Meeting, 2p-Q-14 No. 1 (1997)
p. 265. The technique described in this report omits the first
step, so that a nitride semiconductor film is formed by only the
second through fourth steps. This literature reports that the
threading dislocation density directly above the SiO.sub.2 is
reduced to about 10.sup.5 to about 10.sup.6 cm.sup.-2, as compared
to the about 10.sup.9 to about 10.sup.10 cm.sup.-2 threading
dislocation density within the GaN monocrystalline film which is
formed directly (i.e., in the openings of the SiO.sub.2 masking
layer) on the sapphire substrate.
The above-described techniques for producing a nitride
semiconductor film were expected to reduce the threading
dislocations within the nitride semiconductor film and to improve
the emission characteristics and quality of a nitride semiconductor
light emitting device formed directly on the nitride semiconductor
film.
However, although the above-described nitride semiconductor
film-producing techniques may reduce the threading dislocations
within the resultant nitride semiconductor film, they employ at
least three steps for forming a nitride semiconductor film having
such a reduced threading dislocation density. In addition, it is
necessary to change apparatuses from the first step to the second
step, or from the second step to the fourth step.
In particular, the first conventional technique, which involves the
first through fourth steps as described above, requires two steps
of crystal growth. In general, any regrowth step which is performed
after suspension of a previous growth is accompanied by the problem
of impurity deposition on the crystal surface. This impurity
concern is particularly great for the first conventional technique
because the SiO.sub.2 layer deposited in the second step is
patterned. Moreover, the GaN layer which is utilized as a thick
second layer of nitride semiconductor is grown at a growth
temperature of about 1000.degree. C., thereby leaving the SiO.sub.2
masking pattern which is formed in the third step quite susceptible
to thermal damage. The inventors have discovered through
experimentation that Si or O.sub.2 present in a thermally damaged
masking pattern may unfavorably affect the resultant nitride
semiconductor film.
When a nitride semiconductor light emitting device is produced
directly on a nitride semiconductor film which has been formed by
any conventional nitride semiconductor film-producing technique,
impurities which have been formed as a result of the thermal damage
to the masking pattern may influence an active layer of the nitride
semiconductor light emitting device structure for generating light.
Such influence may result in a decrease in the light emission
efficiency of the individual light emitting devices, low product
reliability due to light emission efficiency variation with respect
to a number of light emitting devices, and/or a low production
yield of nitride semiconductor light emitting devices.
SUMMARY OF THE INVENTION
A nitride semiconductor structure includes: a substrate having a
growth surface, a convex portion and a concave portion being formed
on the growth surface; and a nitride semiconductor film grown on
the growth surface, wherein a cavity is formed between the nitride
semiconductor film and the substrate in the concave portion.
In one embodiment of the invention, the convex portion and the
concave portion are defined by a plurality of parallel grooves.
In another embodiment of the invention, the plurality of grooves
have a width b and a depth h such that b.ltoreq.about 10 .mu.m and
h.gtoreq.0.2.times.b, and adjoining one of the plurality of grooves
are spaced apart from each other so as to retain a distance of
about 20 .mu.m or less between center lines thereof.
In still another embodiment of the invention, the plurality of
grooves have a width b and a depth h such that b.ltoreq.about 10
.mu.m and h.gtoreq.b, and adjoining one of the plurality of grooves
are spaced apart from each other so as to retain a distance of
about 20 .mu.m or less between center lines thereof.
In still another embodiment of the invention, the plurality of
grooves are formed along a <1-100> direction of the nitride
semiconductor.
In still another embodiment of the invention, the grooves are
formed so as to extend along a cleavage plane or an etching stable
plane of the substrate.
In still another embodiment of the invention, the growth surface of
the substrate is composed essentially of a nitride semiconductor,
the plurality of grooves being formed along a <11-20>
direction of the nitride semiconductor.
In another aspect of the invention, there is provided a method for
producing a nitride semiconductor structure including the steps of:
forming a convex and a concave portion or a plurality of grooves on
a growth surface of a substrate; and thereafter growing a nitride
semiconductor film on the growth surface of the substrate so that a
cavity is formed in the concave portion.
In yet another aspect of the invention, there is provided a light
emitting device including: any one of the above nitride
semiconductor structures; and a light emitting structure formed on
the nitride semiconductor structure, the light emitting structure
including an active layer including a nitride semiconductor.
Thus, the invention described herein makes possible the advantages
of (1) providing a nitride semiconductor structure including a
substrate and a high-quality nitride semiconductor film epitaxially
grown thereon, the substrate having a different lattice constant or
a different thermal expansion coefficient from that of the nitride
semiconductor; (2) providing a method for producing such a nitride
semiconductor structure; and (3) providing a light emitting device
incorporating such a nitride semiconductor structure.
These and other advantages of the present invention will become
apparent to those skilled in the art upon reading and understanding
the following detailed description with reference to the
accompanying figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a perspective view illustrating a nitride semiconductor
structure according to Example 1 of the present invention.
FIGS. 2A and 2B are a perspective view and a plan view,
respectively, of a processed substrate according to Example 1 of
the present invention.
FIGS. 3A and 3B are a perspective view and a plan view,
respectively, of a processed substrate according to Example 2 of
the present invention.
FIG. 4 is a plan view illustrating a processed substrate according
to Example 3 of the present invention.
FIGS. 5A and 5B are perspective views illustrating a processed
substrate and a nitride semiconductor according to Example 9 of the
present invention.
FIG. 6 is a cross-sectional view illustrating a laser diode device
structure according to Example 10 of the present invention.
FIG. 7 is a cross-sectional view illustrating a light emitting
diode device structure according to Example 11 of the present
invention.
FIG. 8 is a cross-sectional view illustrating a crystal growth
method according to the present invention.
FIGS. 9A, 9C, 9D, and 9E are cross-sectional views, and FIG. 9B is
a plan view, illustrating a nitride semiconductor structure
according to Example 12 of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Hereinafter, the principles of the present invention will be
described with reference to FIG. 8.
As shown in FIG. 8, a processed substrate 100 includes grooves 115
(note that only one groove is illustrated for the sake of
illustration). As a result, convex portions 114 are formed on the
substrate surface in a complementary manner. A nitride
semiconductor film is formed over the processed substrate 100,
resulting in a nitride semiconductor film 123 which extends upon
the convex portions 114, as well as a nitride semiconductor layer
124 which extends above the groove 115. The groove 115 is not
filled all the way up with the nitride semiconductor 125 that is
deposited at the bottom of the groove 115. Rather, a cavity 116 is
left in the groove 115 which is void of any nitride
semiconductor.
By employing such a grooved substrate, the difference in level
between the groove 115 and the convex portions functions so that
the nitride semiconductor film 123, which is grown directly on the
convex portions 114 adjoining the groove 115, laterally grows from
either side to finally meet in the middle before the groove 115 is
filled up with the nitride semiconductor 125. The lateral growth of
the nitride semiconductor 123 results in the formation of the
nitride semiconductor film 124 above the groove 115. Accordingly,
the cavity 116 which is free of any nitride semiconductor is left
in the groove 115. The nitride semiconductor 124 above the groove
115 (which is a product of the lateral growth of the nitride
semiconductor 123 from the convex portions 114) is not under any
influence from the processed substrate 100 (e.g., stress-induced
strain due to lattice mismatching and/or difference in thermal
expansion coefficient). The presence of the nitride semiconductor
film 124, which substantially escapes the influence from the
substrate, makes it possible to relax the stress-induced strain in
the nitride semiconductor film 123 on the convex portions 114 due
to any lattice mismatching and/or difference in thermal expansion
coefficient between the nitride semiconductor film 124 and the
processed substrate 100. Thus, by utilizing the lateral crystal
growth and effecting crystal growth while controlling the effects
of various stresses, the threading dislocations which extend
vertically from the substrate to the crystal surface can be reduced
as compared to the case of growing crystals on a flat surface.
Furthermore, in the case of forming a thick nitride semiconductor
film, cracks can be minimized due to the reduced stress, unlike in
the prior art.
In order to ensure that this effect permeates the entire nitride
semiconductor film, it is necessary to provide enough grooves 115
per unit area (i.e., so as to have a sufficiently high "groove
density"). The inventors have confirmed through experimentation
that the effects of the invention are attained where adjoining
grooves are spaced apart by about 20 .mu.m or less. In particular,
the strain resulting from various differences between the substrate
and the nitride semiconductor film depends on the groove density.
The effects of strain relaxation can be maximized by increasing the
groove density. On the other hand, if adjoining grooves are spaced
apart by about 20 .mu.m or more, the resultant nitride
semiconductor film may have a threading dislocation density which
is substantially the same as that of a nitride semiconductor film
which is formed on a conventional flat substrate.
Successful formation of the cavities 116 largely depends on the
configuration of the grooves 115 because the cavities 116 are
formed due to the relative readiness of the lateral growth from the
nitride semiconductor film 123 (which is grown directly on the
convex portions 114) as compared to the growth of the nitride
semiconductor 125 within the grooves 115. The inventors have
confirmed that it is preferable that the grooves 115 have a width b
of about 10 .mu.m or less and a depth h such that
h.ltoreq.0.2.times.b.
By employing an increased groove depth h such that h.ltoreq.b,
substantially no crystal growth material is supplied to the bottoms
of the grooves 115, resulting in virtually no deposition of nitride
semiconductor 125. In this case, the formation of the cavities 116
is greatly facilitated irrespective of crystal growth conditions,
and the effects of strain relaxation can be further enhanced.
By forming the grooves 115 on the substrate surface so as to extend
along the <1-100> direction of the nitride semiconductor, the
resultant grooves 115 are oriented perpendicular to a direction
along which the lateral growth occurs most rapidly (hereinafter
this direction will be referred to as the "rapid lateral growth
direction"). As a result, the lateral growth from the nitride
semiconductor film 123 on the convex portions 114 can be further
promoted, thereby enhancing the effects of the present invention.
The inventors have confirmed through experimentation that GaN
semiconductor laterally grows on the substrate so as to extend
along the [11-20] direction of the GaN.
By forming grooves 115 so that their side walls constitute a
cleavage plane of the substrate, the processing of the grooves 115
is facilitated. As a result, it becomes possible to form the
grooves 115 so as to have a steep configuration, thereby providing
for a more defined difference in level between the grooves 115 and
the convex portions 114, while being able to form the grooves 115
so as to have a groove depth which is greater than the groove
width.
In the present specification, grooves will be illustrated as
specific instances of concavity. As used herein, a "groove" is
defined merely as a linear stretch of concavity. A "processed
substrate" is defined as a substrate having convex/concave portions
or grooves formed on its surface. A "cleavage direction" of a
substrate is defined as a direction parallel to the cleavage plane
of the substrate.
Hereinafter, the present invention will be described by way of
specific examples.
EXAMPLE 1
FIG. 1 is a perspective view illustrating a nitride semiconductor
structure having laminated thereon a n-GaN film which has been
produced according to the present example. The nitride
semiconductor structure according to the present example includes a
processed substrate 100, which is obtained by forming grooves 110
on a C plane of a sapphire substrate so as to extend along the
[11-20] direction of the sapphire substrate (i.e., the [1-100]
direction of the GaN film). A GaN buffer layer 120 is formed on
convex portions 114 of the processed substrate 100. A n-GaN film
121 is grown over the entire surface of the processed substrate 100
so as to have a thickness of about 9 .mu.m.
First, a method for producing the processed substrate 100 according
to the present example will be described. The C plane of a sapphire
substrate was used as a growth surface for allowing crystal growth
thereon. FIGS. 2A and 2B are a perspective view and a plan view,
respectively, of the processed substrate 100. The processed
substrate 100 shown in FIGS. 2A and 2B was produced by the
following method.
First, a resist was applied on the surface of the sapphire
substrate, and exposed to UV (ultraviolet) rays. Then, portions
other than the portions which had been cured through the UV
exposure were lifted off, thereby leaving a resist pattern. These
steps can be performed in accordance with a well-known
photolithography technique. Thereafter, the sapphire substrate with
the resist pattern was subjected to a wet etching. The grooves 110
formed on the sapphire substrate surface had a width b of about 6
.mu.m, a depth h of about 2 .mu.m, and a groove pitch L of about 12
.mu.m. The grooves 110 were formed so as to extend along the
[11-20] direction of the sapphire substrate. In general, a sapphire
substrate is known to have the M plane (i.e., {1-100} plane) among
its cleavage planes. Therefore, the C plane of a sapphire substrate
is susceptible to cleavage along the [11-20] direction.
Instead of using the above-described photolithograpy technique, the
processed substrate 100 can be produced by, for example, a scribing
method, a wire saw method, a discharge processing, a sputtering
method, a laser processing, a sandblast processing, or a focus ion
beam (FIB) method to form the grooves 110 on the surface of a
sapphire substrate. Instead of using the above-described wet
etching, a dry etching can be used. For the exposure step, a
holography technique utilizing laser light or electron beam
interference can be used.
According to the present example, the side walls of the grooves 100
constitute the {1-100} cleavage plane (M plane). As will be
appreciated, a cleavage plane is a plane of a given crystal
structure which is likely to be revealed as a result of physical
processing. Alternatively, other cleavage planes may also be used.
For example, for a substrate of a hexagonal system material
(including sapphire substrates), the {1-100} plane (M plane) or the
{01-20} plane (R plane) constitutes a cleavage plane. For a
substrate of a zincblende or diamond structured material, the {110}
plane constitutes a cleavage plane. Grooves 110 can be formed so
that their side walls extend along such plane orientations.
The {1-100} plane of a sapphire substrate is also a plane which is
likely to be revealed as a result of a chemical etching
(hereinafter referred to as an "etching stable plane"). The grooves
110 can be formed in accordance with such an etching stable plane,
whereby similarly steep side walls of the grooves 110 can be
obtained. For a substrate of a hexagonal system material (including
sapphire substrates), the {1-100} plane (M plane), the {11-20}
plane (A plane), the {0001} plane (C plane), or the {01-12} plane
(R plane) constitutes an etching stable plane. For a substrate of a
cubic system material, in particular zincblende or diamond
structured materials, the {111} plane or the {001} plane
constitutes an etching stable plane. Grooves 110 can be formed so
that their side walls extend along such plane orientations.
Next, a process for producing a nitride semiconductor structure by
growing a n-GaN film on a processed substrate 100 by using a MOCVD
apparatus will be described.
The processed substrate 100 as shown in FIG. 1 (or FIG. 2A) was
washed well in an organic solvent, and set in a MOCVD apparatus.
Before growing the n-GaN film 121, the processed substrate 100 was
subjected to a thermal cleaning for about 10 minutes in a H.sub.2
gas flow at a temperature of about 1025.degree. C. Then, the
substrate temperature was lowered to about 550.degree. C., and TMG
(trimethyl gallium) was supplied as a III group material at a rate
of about 10 cc/min, and NH.sub.3 was supplied as a V group material
at a rate of about 5000 cc/min, thereby growing a GaN buffer layer
120 having a thickness of about 20 nm. This method is similar to
known methods for performing epitaxial growth on a sapphire
substrate.
Next, the substrate was heated to a temperature of about
1000.degree. C. TMG was supplied at a rate of about 50 cc/min, and
NH.sub.3 was supplied at a rate of about 5000 cc/min. Furthermore,
SiH.sub.4 (silane) was supplied as a donor impurity, thereby
growing the n-GaN film 121 having a thickness of about 9 .mu.m.
As the thickness of the n-GaN film 121 exceeded about 3 .mu.m, the
grooves 110 on the substrate surface began to be covered, and
therefore flattened, by the n-GaN film 121 while leaving cavities
116 therein. With continued growth, a threading dislocation density
of about 10.sup.7 cm.sup.-2 was obtained as the thickness of the
n-GaN film 121 reached about 9 .mu.m.
A GaN semiconductor which is epitaxially grown on the C plane of a
sapphire substrate is known to be of the following epitaxial
relationship: (0001).sub.sapphire //(0001).sub.GaN and
[1-210].sub.sapphire //[-1010].sub.GaN. Consequently, forming the
grooves 110 along the [11-20] direction of a sapphire substrate is
equivalent to forming the grooves 110 along the [1-100] direction
of GaN. Thus, the grooves 110 according to the present example are
formed along the cleavage direction of the substrate as well as
along the <1-100> direction of the nitride semiconductor
which is grown directly on the substrate.
Since the nitride semiconductor (GaN) laterally grows along the
<11-20> direction of the nitride semiconductor (GaN) in the C
plane of the sapphire substrate, the grooves 110 according to the
present invention are formed along a cleavage direction of the
substrate as well as along a direction perpendicular to the rapid
lateral growth direction of the nitride semiconductor growing on
the substrate. Since the grooves 110 are formed along a cleavage
direction of the substrate, it is easy to process the grooves 110
so that steep side walls of grooves 110 can be provided, whereby a
substantial level difference with the adjoining convex portions 114
can be realized. Since the grooves 110 are formed along a direction
perpendicular to the rapid lateral growth direction of the nitride
semiconductor growing on the substrate, it is easy to form the
cavities 116. As a result, the crystal quality of the resultant
nitride semiconductor film 121 can be enhanced despite
stress-induced strain, and cracks in the nitride semiconductor film
121 can be prevented.
By ensuring that the grooves 110 on the processed substrate 100
have a depth h and a width b such that h.gtoreq.0.2.times.b, it is
possible to relax the stress-induced strain in the nitride
semiconductor film 121 due to any lattice mismatching and/or
difference in thermal expansion coefficient between the n-GaN film
121 and the processed substrate 100. The stress reduction makes it
possible to minimize cracks that may be generated when forming a
thick nitride semiconductor film. If the groove width b is so
large, or if the groove depth h is so small that the condition
h.gtoreq.0.2.times.b is not satisfied, cavities will not be formed
in the grooves 110 because the interior of the grooves 110 will be
filled with the nitride semiconductor film in the initial stage of
growth. As a result, the effects of lateral growth or strain
relaxation cannot be attained. If the groove pitch is increased,
the resultant nitride semiconductor film will have a threading
dislocation density which is substantially the same as that of a
nitride semiconductor film which is formed on a conventional flat
sapphire substrate, as described earlier.
A surface TEM (transmission electron microscopy) was performed in
order to evaluate the density of threading dislocations appearing
on the surface of the n-GaN film 121 shown in FIG. 1. The results
indicated that the threading dislocation density on the surface of
the grown film had been reduced to about 10.sup.7 cm.sup.-2, which
is substantially the same as, or not significantly higher than, the
threading dislocation density levels reported for any conventional
technique.
Under the prior art, the masking pattern used for the selective
growth is likely to be thermally damaged during the second growth
stage of a nitride semiconductor, so that the component elements of
the masking pattern may serve as impurities that affect the grown
nitride semiconductor film. On the contrary, the thin growth film
layer produced according to the present invention does not contain
any component elements to serve as such impurities. The inventors
conducted a photoluminescence (PL) measurement of the grown nitride
semiconductor film to calculate an intensity ratio between the peak
intensity of the near band edge associated with the grown nitride
semiconductor film (a single GaN film) and the intensity from a
deep band level due to impurities. As a result, it was indicated
that the intensity ratio obtained according to the present example
had been improved by one order of magnitude or more than the
intensity ratio obtained by a conventional technique employing a
masking pattern. This indicates the excellent quality of the
nitride semiconductor film grown according to the present
example.
Although the grooves 110 on the processed substrate 100 illustrated
above had a pitch L of about 12 .mu.m, the pitch L can be further
reduced for a higher groove density, thereby minimizing the
threading dislocation density. Although the grooves 110 illustrated
above had a width b of about 6 .mu.m, it is also possible to employ
a smaller groove width. Although the groove pitch according to the
present example is illustrated as constant, the groove pitch does
not need to be constant as long as the intervals between grooves is
maintained at about 20 .mu.m or less.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 2
Example 2 is a modification of Example 1, where the same
configuration as that of Example 1 is employed except that a
different groove pattern is formed on the sapphire substrate. FIGS.
3A and 3B illustrate a processed substrate 100 formed according to
Example 2 of the present invention.
The processed substrate 100 according to the present example is
obtained by forming grooves 111 on the C plane of the sapphire
substrate so as to extend along the [11-20] direction and the
[-2110] direction of the sapphire substrate, with a n-GaN film
being grown directly on the processed substrate 100 so as to have a
thickness of about 8 .mu.m. Hereinafter, the processed substrate
100 and the n-GaN film grown thereon according to the present
example will be described.
The C plane of a sapphire substrate was used as a growth surface
for allowing crystal growth thereon. The processed substrate 100
shown in FIGS. 3A and 3B was produced by using a FIB technique. The
grooves 111 formed on the sapphire substrate surface had a width b
of about 1 .mu.m, a depth h of about 3 .mu.m, and a groove pitch L
of about 3 .mu.m. The grooves 111 were formed so as to extend along
the [11-20] direction or the [-2110] direction of the sapphire
substrate.
Next, a n-GaN film was grown on the processed substrate 100 by
using a MOCVD apparatus as follows. First, the processed substrate
100 as shown in FIG. 3A was washed well in an organic solvent, and
set in a MOCVD apparatus. Before growing a n-GaN film, the
processed substrate 100 was subjected to a thermal cleaning for
about 10 minutes in a H.sub.2 gas flow at a temperature of about
1025.degree. C. Then, the substrate temperature was lowered to
about 500.degree. C., and TMA (trimethyl aluminum) was supplied as
a III group material at a rate of about 20 cc/min, and NH.sub.3 was
supplied as a V group material at a rate of about 5000 cc/min,
thereby growing an AlN buffer layer having a thickness of about 50
nm. This method is similar to known methods for performing
epitaxial growth on a sapphire substrate.
Next, the substrate was heated to a temperature of about
1000.degree. C. TMG was supplied at a rate of about 50 cc/min, and
NH.sub.3 was supplied at a rate of about 5000 cc/min. Furthermore,
SiH.sub.4 (silane) was supplied as a donor impurity, thereby
growing a n-GaN film having a thickness of about 8 .mu.m.
Like in Example 1, as the thickness of the n-GaN film exceeded
about 2 .mu.m, the grooves 111 on the substrate surface began to be
covered, and therefore flattened, by the n-GaN film while leaving
cavities therein. With continued growth, a threading dislocation
density of about 10.sup.5 cm.sup.-2 to about 10.sup.6 cm.sup.-2 was
obtained as the thickness of the n-GaN film reached about 8 .mu.m.
As in Example 1, the PL intensity of the near band edge associated
with the grown nitride semiconductor film proved very intense,
whereas the intensity from a deep band level due to impurities was
extremely small. Thus, a high-quality single GaN film was formed as
in Example 1.
The relative depth of the grooves 111 according to the present
example is larger than that of the grooves 110 in Example 1. By
employing a groove depth which is larger than the groove width, the
supply of source gas into the grooves 111 during the vapor
deposition step becomes extremely scarce, so that virtually no
crystal growth occurs within the grooves 111.
According to the present example, the grooves 111 are formed along
a plurality of directions. The grooves 111 extending along the
<11-20> of the sapphire substrate can take any of three
directions in the C plane of the sapphire substrate, i.e., [11-20],
[-2110], and [1-210]. When a nitride semiconductor is grown in a C
axis alignment with respect to a sapphire substrate as in the
present example, three directions also exist in the C plane of the
nitride semiconductor. The grooves 111 on the processed substrate
100 according to the present example were formed along two selected
ones of these three directions. As a result, the GaN which had been
grown on the processed substrate 100 underwent an enhanced lateral
growth along directions which are respectively perpendicular to the
selected two directions. The pitch of the grooves 111 according to
the present example is reduced to half of the pitch of the groove
110 for an increased groove density. Accordingly, the threading
dislocation density according to the present example was improved
by about one order of magnitude than that obtained in Example
1.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 3
Example 3 is a modification of Example 2, where the same
configuration as that of Example 2 is employed except that a
different groove pattern is formed on the sapphire substrate. In
the present example, grooves 112 are formed along all of the three
directions that equivalently correspond to the <11-20>
direction of the sapphire substrate as described in Example 2.
Accordingly, all of the three <1-100> directions of the
nitride semiconductor are selected, too.
FIG. 4 illustrates a processed substrate 100 formed according to
Example 3 of the present invention.
The processed substrate 100 according to the present example is
obtained by forming grooves 112 on the C plane of the sapphire
substrate so as to extend along the [11-20] direction, the [-2110]
direction, and the [1-210] direction of the sapphire substrate,
with a n-GaN film being grown on the processed substrate 100 so as
to have a thickness of about 8 .mu.m. Hereinafter, the processed
substrate 100 produced according to the present example will be
described.
The C plane of a sapphire substrate was used as a growth surface
for allowing crystal growth thereon. The processed substrate 100
shown in FIG. 4 was produced by using a FIB technique as in Example
2. The grooves 112 formed on the sapphire substrate surface had a
width b of about 1 .mu.m, a depth of about 2 .mu.m, and a groove
pitch L of about 4 .mu.m. The grooves 112 were formed so as to
extend along the [11-20] direction, the [-2110] direction, or the
[1-210] direction of the sapphire substrate.
According to the present example, the GaN which had been grown on
the processed substrate 100 underwent a lateral growth along
directions which are respectively perpendicular to all of the three
directions. Since the depth of the grooves 112 is larger than the
width of the grooves 112, as in Example 2, the nitride
semiconductor structure of the present example is substantially
immune to the stress-induced strain due to the lattice mismatching
and/or difference in thermal expansion coefficient between the
substrate and the nitride semiconductor film.
The threading dislocation density and PL luminescence results
according to the present example were substantially the same as
those obtained in Example 2. Thus, a high-quality single GaN film
was formed as in Example 2, while similarly suppressing crack
generation.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 4
Example 4 is a modification of Example 1 or 2, where the same
configuration as that of Example 1 or 2 is employed except that the
M plane of a sapphire substrate is employed instead of the C plane,
and that certain groove directions are chosen. Hereinafter, a
processed substrate produced according to the present example will
be described.
A GaN semiconductor which is epitaxially grown on the M plane of a
sapphire substrate is of the following epitaxial relationship:
(01-10).sub.sapphire //(01-13).sub.GaN and [0001].sub.sapphire
//[2-1-10].sub.GaN. Consequently, forming a groove whose {1-100}
plane extends along the [0001] direction of the M plane of the
sapphire substrate is equivalent to forming the groove along the M
plane ({1-100}), which is a cleavage plane of the sapphire
substrate. Similarly, forming a groove whose {1-100} plane extends
along the [2-1-10] direction of the M plane of the sapphire
substrate is equivalent to forming the groove along a direction
perpendicular to the direction of lateral growth of the GaN which
is grown on the M plane of the sapphire substrate. Grooves are
formed on the M plane of the sapphire substrate along at least one
of these two directions, thereby producing a processed
substrate.
Where the grooves are formed along only one of the above two
directions, the present example constitutes a modification of
Example 1. Where the grooves are formed along both directions, the
present example constitutes a modification of Example 2.
Processed substrates were produced with grooves having a width b of
about 2 .mu.m, a depth h of about 3 .mu.m, and a groove pitch L of
about 5 .mu.m, and a nitride semiconductor film was formed so as to
have a thickness of about 10 .mu.m. Furthermore, two subspecies of
the processed substrate were formed so as to include grooves
extending along either one direction or both directions. Both
subspecies exhibited a threading dislocation density of about
10.sup.6 cm.sup.-2 to about 10.sup.7 cm.sup.-2. This threading
dislocation density range was about the same as that reported for
any conventional technique utilizing a masking pattern. According
to the present example, the direction of the cleavage plane of the
processed substrate does not coincide with a direction
perpendicular to the direction of lateral growth of the nitride
semiconductor, which is considered as responsible for the threading
dislocation density which is one order of magnitude higher than
that obtained in Example 2 or 3.
As in Example 1, the PL intensity of the near band edge associated
with the grown nitride semiconductor film was very intense, whereas
the intensity from a deep band level due to impurities was
extremely small. Crack generation was similarly suppressed,
too.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 5
Example 5 is a modification of Examples 1 to 3, where the same
configuration as that of Examples 1 to 3 is employed except that
the A plane of a sapphire substrate is employed instead of the C
plane. Hereinafter, a processed substrate produced according to the
present example will be described.
A GaN semiconductor which is epitaxially grown on the C plane of a
sapphire substrate is of the either one of the following two
epitaxial relationships (i) or (ii), depending on the crystal
growth conditions of the nitride semiconductor:
(i) (2-1-10).sub.sapphire //(0001).sub.GaN, [0001].sub.sapphire
//[2-1-10].sub.GaN, and [01-10].sub.sapphire //[01-10].sub.GaN ;
or
(ii) (2-1-10).sub.sapphire //(0001).sub.GaN, [0001].sub.sapphire
//[01-10].sub.GaN, and [01-10].sub.sapphire //[2-1-10].sub.GaN.
Under epitaxial relationship (i), grooves are formed along the
[0001] direction, a direction at an angle of 32.4.degree. with
respect to the [0001] direction, or the [01-10] direction. Grooves
which are formed along the first two directions have side walls
corresponding respectively to the M plane (the {1-100} plane) and
the R plane (the {01-12} plane), both of which are cleavage faces
of a sapphire substrate. The third direction is perpendicular to
the lateral growth of GaN which is grown on the A plane of a
sapphire substrate. Grooves are formed on the A plane of a sapphire
substrate so as to extend along each one of these directions, or
any combination thereof, thereby forming a processed substrate.
Where the grooves are formed along only one of the above three
directions, the present example constitutes a modification of
Example 1. Where the grooves are formed along any combination of
these directions, the present example constitutes a modification of
Example 2 or 3. The same effects as in Example 4 are attained in
the present example (epitaxial relationship (i)), while similarly
suppressing crack generation, by utilizing epitaxial relationship
(i) and the A plane of a sapphire substrate.
Under epitaxial relationship (ii), grooves are formed along the
[0001] direction, or a direction at an angle of 32.4.degree. with
respect to the [0001] direction. Grooves which are formed along the
former direction have side walls corresponding to the M plane (the
{1-100} plane), which is a cleavage face of a sapphire substrate,
and this direction is perpendicular to the lateral growth of GaN
which is grown on the A plane of the sapphire substrate.
Accordingly, the same effects as in Example 1 (which utilized the C
plane of the sapphire substrate) are obtained according to the
present example (epitaxial relationship (ii)). On the other hand,
grooves which are formed along the latter direction have side walls
corresponding to the R plane (the {01-12} plane), which is a
cleavage plane of a sapphire substrate. Grooves are formed on the A
plane of a sapphire substrate so as to extend along either one, or
both, of these directions.
Where the grooves are formed along only one direction, the present
example constitutes a modification of Example 1 as shown in FIGS.
2A and 2B. Where the grooves are formed along both directions, the
present example constitutes a modification of Example 2 or 3. The
same effects as in Example 1 are attained in the present example,
while similarly suppressing crack generation, by utilizing
epitaxial relationship (ii) and the A plane of a sapphire
substrate.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 6
Example 6 is a modification of Example 1, where the same
configuration as that of Example 1 is employed except that the R
plane of a sapphire substrate is employed instead of the C plane.
Hereinafter, a processed substrate produced according to the
present example will be described.
A GaN semiconductor and the R plane of a sapphire substrate are of
the following epitaxial relationship: (001-2).sub.sapphire
//(2-1-10).sub.GaN and [2-1-10].sub.sapphire //[01-10].sub.GaN.
Consequently, forming a groove whose {1-100} plane extends along
the [2-1-10] direction of the R plane of the sapphire substrate is
equivalent to forming the groove along the M plane ({1-100}), which
is a cleavage plane of the sapphire substrate. Grooves are formed
on the R plane of the sapphire substrate along this direction,
thereby producing a processed substrate. The threading dislocation
density obtained according to the present example is about 10.sup.7
cm.sup.-2 to about 10.sup.8 cm.sup.-2, which is substantially the
same as, or not significantly higher than, the threading
dislocation density levels reported for any conventional technique
using a masking pattern. However, the PL intensity of the deep band
level due to impurities was similar to that obtained in Example 1.
Crack generation was similarly suppressed, too.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 7
Example 7 is a modification of Example 1 to 3, where the same
configuration as that of Example 1 to 3 is employed except that the
(0001) plane of a 6H--SiC substrate is employed instead of the C
plane of a sapphire substrate. Hereinafter, a processed substrate
produced according to the present example will be described.
In the case where a GaN semiconductor which is epitaxially grown on
the (0001) plane of a 6H--SiC substrate, the GaN semiconductor and
the (0001) plane of the 6H--SiC substrate are of the following
epitaxial relationship: (0001).sub.6H--SiC //(0001).sub.GaN, and
[01-10].sub.6H--SiC //[01-10].sub.GaN.
Accordingly, grooves are formed along the [2-1-10] direction or the
[01-10] direction of the (0001) plane of the 6H--SiC substrate.
Grooves which are formed along the former direction have side walls
corresponding to the {1-100} plane, which is a cleavage plane of a
(0001) plane 6H---SiC substrate. The latter direction is
perpendicular to the lateral growth of GaN which is grown on the
(0001) plane of the 6H--SiC substrate. Grooves are formed on the
(0001) plane of the 6H--SiC substrate so as to extend along either
one, or both, of these directions, thereby forming a processed
substrate.
Where the grooves are formed along only one direction, the present
example constitutes a modification of Example 1. Where the grooves
are formed along both directions, the present example constitutes a
modification of Example 2 or 3. The same effects as in Example 4
are attained in accordance with the processed substrate of the
present example.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 8
Example 8 is a modification of Examples 1 to 3, where the same
configuration as that of Examples 1 to 3 is employed except that
the (111) plane of a MgAl.sub.2 O.sub.4 (magnesia spinel) substrate
is employed instead of the C plane of a sapphire substrate.
Hereinafter, a processed substrate produced according to the
present example will be described.
In the case where a GaN semiconductor which is epitaxially grown on
the (111) plane of a MgAl.sub.2 O.sub.4 substrate, the GaN
semiconductor and the (111) plane of the MgAl.sub.2 O.sub.4
substrate are of the following epitaxial relationship:
(111).sub.MgAl2O4 //(0001).sub.GaN, [-110].sub.MgAl2O4
//[2-1-10].sub.GaN, and [11-2].sub.MgAl2O4 //[01-10].sub.GaN.
Accordingly, with regard to the (111) plane of the MgAl.sub.2
O.sub.4 substrate, grooves whose {100} plane extends along the
[-110] direction or grooves which extend along the [11-2] direction
are formed. Grooves which are formed along the former direction
have side walls corresponding to the {100} plane, which is a
cleavage plane of a (111) plane MgAl.sub.2 O.sub.4 substrate. The
latter direction is perpendicular to the lateral growth of GaN
which is grown on the (111) plane of the MgAl.sub.2 O.sub.4
substrate. Grooves are formed on the (111) plane of the MgAl.sub.2
O.sub.4 substrate so as to extend along either one, or both, of
these directions, thereby forming a processed substrate.
Where the grooves are formed along only one direction, the present
example constitutes a modification of Example 1. Where the grooves
are formed along both directions, the present example constitutes a
modification of Example 2 or 3. The same effects as in Example 4
are attained in accordance with of the processed substrate of the
present example.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 9
FIG. 5A is a perspective view illustrating a GaN film which has
been produced according to the present example. FIG. 5B is a
perspective view illustrating a substrate for growing thereon a GaN
film according to the present example. The nitride semiconductor
structure according to the present example includes a processed
substrate 100, which is obtained by forming random convex/concave
portions 113 on the C plane of a sapphire substrate. A GaN layer
122 is formed on the processed substrate 100 so as to have a
thickness of about 30 .mu.m. Hereinafter, the processed substrate
100 according to the present example, as well as a method for
growing a thick GaN film thereon by using a HVPE method, will be
described.
The C plane of a sapphire substrate was used as a growth surface
for allowing crystal growth thereon. The processed substrate 100
shown in FIG. 5B is obtained by forming random convex/concave
portions 113 on a sapphire substrate by using an Ar.sup.+ ion
milling method. The convex/concave portions 113 had an average
level difference of about 3 .mu.m (as determined by surface
roughness measurement), and had an average undulation pitch
(analogous to a wavelength of the undulations) of about 13
.mu.m.
Next, the n-GaN film 122 was grown on the processed substrate 100
having the convex/concave portions 113 formed thereon in the
aforementioned step. First, the processed substrate 100 was washed
well in an organic solvent, and set in a HVPE apparatus. Before
growing the n-GaN film 122, the processed substrate 100 was
subjected to a thermal cleaning for about 10 minutes in a H.sub.2
gas flow at a temperature of about 1025.degree. C. A gas obtained
by mixing NH.sub.3 gas (at a rate of about 2000 cc/min) and a
carrier H.sub.2 gas (at a rate of about 10000 cc/min) was used as a
V group gas. As a III group gas, a gas obtained by mixing a carrier
H.sub.2 gas (at a rate of about 1000 cc/min) with a III group
chloride which had been obtained as an Ga--HCl reaction product by
feeding HCl gas (at a rate of about 100 cc/min) to a Ga metal
material which had previously been maintained in a HVPE apparatus
at about 700.degree. C. was used. In order to begin crystal growth
of the GaN film 122, the V group gas and the III group gas were
supplied in a HVPE growth reactor in which the processed substrate
100 had been set. As a result, the GaN film 122 as shown in FIG. 5A
was formed so as to have a thickness of about 30 .mu.m.
As the thickness of the GaN film 122 exceeded about 5 .mu.m, the
convex/concave portions 113 on the surface of the processed
substrate 100 began to be covered, and therefore flattened, by the
GaN film 122 while leaving cavities therein. With continued growth,
a threading dislocation density of about 10.sup.8 cm.sup.-2 was
obtained as the thickness of the GaN film 122 reached about 30
.mu.m. An observation of the substrate surface by means of an
optical microscope revealed no cracks. The threading dislocation
density within the GaN film obtained according to the present
example is substantially the same as the threading dislocation
density levels reported for any conventional technique that does
not use a masking pattern. However, the GaN film grown on the
processed substrate 100 according to the present example had
virtually no cracks generated on the surface of the film in the
initial growth stage, as opposed to that obtained by any
conventional technique for growing a thick film directly on a
sapphire substrate. Thus, the grown thick film with a smaller
thickness obtained the same threading dislocation density.
Although Examples 1 to 9 above illustrated GaN as a nitride
semiconductor to be grown in the structure, it is also applicable
to employ other nitride semiconductors, e.g., Al.sub.x Ga.sub.y
In.sub.1-x-y N (where 0.ltoreq.x.ltoreq.1 and 0.ltoreq.y.ltoreq.1),
or any material obtained by substituting any one of the elements of
Al.sub.x Ga.sub.y In.sub.1-x-y N (where 0.ltoreq.x.ltoreq.1 and
0.ltoreq.y.ltoreq.1), with an element selected from B, Cr, V, Ti,
Nb, Ta, Zr, Sc, Tl, Gd, La, As, P, Sb, etc., as long as the
substituted element accounts for about 10% or less of the entire
material.
Thus, according to the present example, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique.
EXAMPLE 10
FIG. 6 illustrates an exemplary LD device structure produced
according to Example 10 of the present invention. The present
example illustrates a LD device. structure which, as a light
emitting device, is formed directly on a processed substrate 200
(having a n-GaN film formed thereon) produced according to Example
2.
Hereinafter, a method for producing the semiconductor light
emitting device according to the present example will be
described.
First, a nitride semiconductor structure (which is herein defined
as including both a processed substrate and a n-GaN film grown
thereon) 200 was placed in a MOCVD apparatus, and subjected to a
thermal cleaning at about 1050.degree. C. Then, among various
material gases which the MOCVD apparatus was equipped with,
NH.sub.3 gas was used as a V group material gas; TMG (trimethyl
gallium) gas was used as a III group material; and SiH.sub.4
(silane) gas was used as a donor impurity, in order to grow a
Si-doped n-GaN layer 201 on the nitride semiconductor structure so
as to have a thickness of about 3 .mu.m at a growth temperature of
about 1000.degree. C.
Next, in order to form a second layer (cladding layer), NH.sub.3,
TMG and TMA gases were used as material gases, and SiH.sub.4 gas
was used as a donor impurity, thereby growing a Si-doped Al.sub.0.1
Ga.sub.0.9 N layer 202 so as to have a thickness of about 0.4
pm.
Next, in order to form a third layer (optical guide layer),
NH.sub.3 and TMG gases were used as material gases, and SiH4 gas
was used as a donor impurity, thereby growing a Si-doped GaN layer
203 so as to have a thickness of about 0.1 .mu.m.
In order to form a fourth layer (multiple quantum well layer),
NH.sub.3, TMG and TMI (trimethyl indium) gases were used as
material gases, and SiH.sub.4 gas was used as a donor impurity,
thereby creating a five-fold Si-doped n-multiple quantum well layer
204, where each cycle consisted of an In.sub.0.2 Ga.sub.0.8 N
(thickness: about 2 nm) and an In.sub.0.05 Ga.sub.0.95 N
(thickness: about 3 nm). Furthermore, a p-Al.sub.0.2 Ga.sub.0.8 N
evaporation prevention layer 205 (thickness: about 30 nm) was grown
in order to prevent evaporation of In in the quantum well active
layer 204 during growth of the nitride semiconductor layer
immediately above the active layer.
Next, in order to form a fifth layer (optical guide layer),
NH.sub.3 and TMG gases were used as material gases, and EtCp.sub.2
Mg (bisethylcyclopentadienylmagnesium) gas was used as an acceptor
impurity, thereby growing a Mg-doped p-GaN layer 206 so as to have
a thickness of about 0.1 .mu.m.
Next, in order to form a sixth layer (cladding layer), NH.sub.3,
TMG, and TMA gases were used as material gases, and EtCp.sub.2 Mg
gas was used as an acceptor impurity, thereby growing a Mg-doped
p-Al.sub.0.1 Ga.sub.0.9 N layer 207 so as to have a thickness of
about 0.4 .mu.m.
Lastly, in order to form a seventh layer (contact layer), NH.sub.3,
TMG, and TMA gases were used as material gases, and EtCp.sub.2 Mg
gas was used as an acceptor impurity, thereby growing a Mg-doped
p-GaN layer 208 so as to have a thickness of about 0.5 .mu.m.
Furthermore, a positive electrode 210, and a negative electrode 209
were formed on the Mg-doped pGaN layer 208 and the Si-doped GaN
layer 201, respectively, thereby accomplishing the LD device. As
for the method of laminating the n layers and the p layers of this
LD device structure, it is also applicable to form the p layers
first, and then form the active layer and the n layers.
Although the processed substrate 200 of the above-described LD
device incorporates a n-GaN film having a flat surface as produced
according to Example 2, it will be appreciated that a GaN film
produced according to Example 1 or any of Examples 3 to 9 may
alternatively be adopted. Furthermore, the GaN film formation
according to any of Examples 1 to 9 and the LD device production
according to the present example may be consecutively performed in
the same apparatus. Alternatively, it is also possible to employ
the GaN film obtained according to the respective Examples by
itself, i.e., after removing the processed substrate.
A high temperature acceleration test using a 2 mW optical output
power was conducted for the LD device produced according to Example
10 in an atmosphere at about 50.degree. C., as a result of which
the inventors confirmed that the LD device had a continuous
operation lifetime of about 8000 hours (as converted into use under
room temperature). This continuous operation lifetime provides more
than about 20% improvement over that of a LD device produced by a
conventional method which was similarly subjected to a high
temperature acceleration test. Such high reliability of the LD
device according to the present invention owes to the decrease in
threading dislocation density and the elimination of unwanted
impurities according to the present example.
EXAMPLE 11
FIG. 7 illustrates an exemplary structure of a LED device produced
according to Example 11 of the present invention. The present
example illustrates a LED device structure which is constructed on
a nitride semiconductor structure according to Example 1 by using a
MBE apparatus.
A processed substrate 300 with a n-GaN film grown thereon, as
produced according to Example 1, was placed in a MBE apparatus. A
n-GaN first layer 301 was formed so as to have a thickness of about
2 .mu.m. Then, an In.sub.0.45 Ga.sub.0.55 N second layer (single
quantum well layer) 302 was formed so as to have a thickness of
about 4 nm. Furthermore, a p-Al.sub.0.l Ga.sub.0.9 N evaporation
prevention layer 303 (thickness: about 100 nm) was grown in order
to prevent evaporation of In in the quantum well active layer 302
during growth of the nitride semiconductor layer immediately above
the active layer. Lastly, a p-GaN third layer (contact layer) 304
was formed so as to have a thickness of about 0.4 .mu.m.
Furthermore, a positive electrode 306, and a negative electrode 305
were formed on the Mg-doped p-GaN third layer (contact layer) 304
and the n-GaN first layer 301, respectively, thereby accomplishing
the LED device.
The electron-photon conversion efficiency of the LED device
products produced according to the present example was measured. As
a result, it was confirmed that products having an electron-photon
conversion efficiency of about 5% or higher (which is considered as
a satisfactory level) accounted for about 88% or more of the entire
wafer, indicative of about 13% improvement in the LED device
production yield from conventional techniques. Furthermore, the LED
device of the present invention was subjected to a reliability test
after 1000 hours of operation, which exhibited luminescence of
about 97% or more of the level attained at the beginning of the
test. Thus, it was also confirmed that the LED device has practical
reliability. Such high reliability of the LED device according to
the present invention owes to the decrease in threading dislocation
density, the elimination of unwanted impurities, and the prevention
of cracks according to the present example.
Although the processed substrate 300 of the above-described LED
device incorporates a n-GaN film as produced according to Example
1, it will be appreciated that a GaN film produced according to any
of Examples 2 to 9 may alternatively be adopted. Furthermore, the
GaN film formation according to any of Examples 1 to 9 and the LED
device production according to the present example may be
consecutively performed in the same apparatus. Alternatively, it is
also possible to employ the GaN film obtained according to the
respective Examples by itself, i.e., after removing the processed
substrate.
EXAMPLE 12
FIG. 9A is a cross-sectional view illustrating the structure of a
GaN film 405 which is produced according to Example 12 of the
present invention. The broken line in FIG. 9A corresponds to the
shape of grooves 403 in FIG. 9C. The device according to the
present example includes a GaN buffer layer 401, a GaN layer 402,
and a GaN film 405 grown on a sapphire substrate 400. The grooves
403 are formed on the GaN layer 402. Cavities 404 are formed so as
to correspond to the grooves 403.
A method for producing the device of the present example will be
described. The C plane of a sapphire substrate 400 was used as a
growth surface for allowing crystal growth thereon. The sapphire
substrate 400 was placed in the reactor of a MOCVD apparatus, and
subjected to a thermal cleaning for about 10 minutes in a H.sub.2
atmosphere flow at a temperature of about 1100.degree. C. Then, TMG
was supplied as a III group material and NH.sub.3 was supplied as a
V group material in the growth reactor, thereby growing the GaN
buffer layer 401 having a thickness of about 30 nm at a growth
temperature of about 550.degree. C. Instead of a GaN buffer 401, an
AlN buffer layer may alternatively be used. These steps are part of
well-known techniques for nitride semiconductor crystal growth.
After the GaN buffer layer 401 is grown, the substrate temperature
is increased to about 1050.degree. C. in order to grow the GaN
layer 402 so as to have a thickness of about 3 .mu.m. Next, the
substrate on which the GaN layer 402 had been grown (hereinafter
referred to as the "sapphire substrate with GaN") is removed from
the MOCVD apparatus reactor, and the grooves 403 were formed on the
GaN layer 402 so to extend along the <11-20> direction of the
GaN crystal by using a FIB method. The side walls of the grooves
403 constitute the {1-100} cleavage plane of the GaN crystal. FIGS.
9B and 9C are a plan view and a cross-sectional view, respectively,
illustrating the grooves 403 formed in this step. As shown in FIGS.
9A to 9C, the GaN film is formed on the growth surface of the
substrate.
The grooves 403 had a width b of about 5 .mu.m, a depth h of about
2 .mu.m, and a groove pitch L of about 10 .mu.m. The width b and
the depth h of the grooves 403 satisfied at least
h.gtoreq.0.2.times.b.
The GaN film 405 was grown on the GaN layer 402 (having the grooves
403 formed thereon through the above steps) so as to have a
thickness of about 200 .mu.m by using a HVPE method. A method for
producing the GaN film 405 will be described below.
First, the sapphire substrate with GaN was washed well in an
organic solvent, and set in a HVPE apparatus. A gas obtained by
mixing NH.sub.3 gas (at a rate of about 2000 cc/min) and a carrier
H.sub.2 gas (at a rate of about 10000 cc/min) was used as a V group
gas. As a III group gas, a gas obtained by mixing a carrier H.sub.2
gas (at a rate of about 1000 cc/min) with a III group chloride
which had been obtained as a Ga--HCl reaction product by feeding
HCl gas (at a rate of about 100 cc/min) to a Ga metal material
which had previously been maintained in a HVPE apparatus at about
700.degree. C. was used. In order to begin crystal growth of the
GaN film 405, the V group gas and the III group gas were supplied
in a HVPE growth reactor. As a result, the GaN film 405 was formed
so as to have a thickness of about 200 .mu.m. The GaN film 405 was
grown to attain a flat configuration, with the grooves 403
completely buried. An observation of the surface of the GaN film
405 by means of an optical microscope revealed no cracks. The
threading dislocation density within the GaN film 405 obtained
according to the present example was about 10.sup.6 cm.sup.-2 to
about 10.sup.7 cm.sup.-2. Since no masking pattern such as
SiO.sub.2 was used, as in the above examples, unwanted impurities
were prevented from being mixed.
Although the grooves 403 had a rectangular cross section, {1-101}
facets were spontaneously formed (FIG. 9D) during the growth of the
GaN film 405. This is ascribable to the fact that the {1-101} plane
has a slower GaN crystal growth rate than other plane orientations.
The reason for forming the grooves 403 along the <11-20>
direction according to the present example is to allow the {1-101}
plane of the nitride semiconductor crystal (in particular GaN
crystal) to be formed as side walls of the grooves 403 because the
{1-101} plane has a slower crystal growth rate than the {0001}
plane. Alternatively, any direction other than the <11-20>
direction can be used so long as it allows the {1-101} plane of the
nitride semiconductor crystal to be formed as side walls of the
grooves 403.
Alternatively, a {11-2i} (where 0.ltoreq.i.ltoreq.3) plane may be
used as a facet having a slower crystal growth rate than that the
{0001} plane, instead of the aforementioned {1-101} facet. In order
to allow a {11-2i} (where 0<i.ltoreq.3) plane to be formed as
side walls of the grooves 403, the grooves can be formed so as to
extend along the <1-100> direction on a nitride semiconductor
film having the {0001} plane. Any method other than the method of
forming the grooves along the <11-20> direction can be used
so long as it allows a {11-2i} (where 0.ltoreq.i.ltoreq.3) plane to
be formed as side walls of the grooves 403.
The inventors confirmed the following facts upon observing the
process of the grooves 403 being buried in the GaN film 405. At
first, the grooves 403 on the GaN layer 402 at first appeared to
become deeper with the growth of the GaN film 405 because the
crystal growth on the {0001} plane occurs faster than the crystal
growth on the {1-101} plane so that the growth was promoted along
the growth axis without allowing the grooves 403 to be buried (FIG.
9D). It is presumable that, since GaN has a slower crystal growth
rate (i.e., it has a long surface diffusion distance) on the
{1-101} plane than on the other planes, any Ga atoms which came
onto the {1-101} plane are ejected (before they can be deposited as
GaN) to the {0001} plane, where the Ga atoms bind with N atoms to
form GaN.
As the crystal growth of the GaN film 405 proceeds, the growth
surface on the {0001} plane is reduced until grooves or undulation
having a triangular configuration, surrounded by {1-101} facets,
are formed as shown by the solid line in FIG. 9D. The broken line
in FIG. 9A corresponds to the grooves 403 in FIG. 9C. With further
growth of GaN crystal, the GaN crystal begins to grow on the
{1-101} facets because the Ga atoms, which has been ejected onto
the {0001} plane, cannot go anywhere but remain on the {1-101}
facets. This is because the growth on the {0001} plane is along the
growth axis, whereas the growth on the {1-101} facets is along a
pseudo-lateral direction with respect to the growth axis. The
grooves 403 begin to be buried with the growth on the {1-101}
facets. However, the grooves 403 have become deeper than when they
were initially formed due to the growth along the growth axis which
had occurred before the {1-101} facets buried the grooves 403, so
that now it is difficult for the material gases to enter into the
grooves 403. Moreover, the GaN depositions which have laterally
grown from both sides of each groove 403 meet in the middle over
the groove 403, but do not completely combine with each other due
to a slight difference in crystal orientation, thereby leaving a
cavity 404 as shown in FIG. 9E. The broken line and the
dot-and-dash line in FIG. 9E respectively correspond to the
configurations shown in FIGS. 9C and 9D.
The cavities 404 which have been thus formed provide relaxation of
strain as in the above-described examples. The reduced threading
dislocation density of the structure according to the present
example is not only due to the presence of the cavities 404 but
presumably because the threading dislocations are deflected in a
lateral direction from the growth axis direction at the {1-101}
facets as the cavities 404 are buried through lateral growth, so
that fewer threading dislocations reach the outermost surface of
the GaN film 405.
If the width b and the depth h of the grooves 403 formed on the
nitride semiconductor film satisfy h.gtoreq.b, the grooves 403 are
sufficiently deep to prevent the material gases from reaching the
bottoms of the grooves 403, so that cavities are formed even
without burying the grooves 403. Therefore, in the case where the
grooves 403 are sufficiently deep, the cavities are formed in
accordance with Example 2 or 3 rather than with the present
example.
Thus, according to the present invention, it is possible to
epitaxially grow a high-quality nitride semiconductor film on a
substrate having a different lattice constant or a different
thermal expansion coefficient from that of the nitride
semiconductor, without employing a complicated two-phase growth
technique, so that the nitride semiconductor film has a low
threading dislocation density, includes substantially no cracks
when formed to a large thickness, and are substantially free of
unwanted impurities. In addition, a LED or LD having a very high
emission efficiency can be produced with a high yield by
constructing a light emitting device structure on the high-quality
nitride semiconductor film grown by the nitride semiconductor film
production method of the present invention.
Various other modifications will be apparent to and can be readily
made by those skilled in the art without departing from the scope
and spirit of this invention. Accordingly, it is not intended that
the scope of the claims appended hereto be limited to the
description as set forth herein, but rather that the claims be
broadly construed.
* * * * *